Summer Fish Research Articles

Changes in expression of appetite-regulating hormones in the cunner (Tautogolabrus adspersus) during short-term fasting and winter torpor

Feeding in vertebrates is controlled by a number of appetite stimulating (orexigenic, e.g. orexin and neuropeptide Y, NPY) and appetite suppressing (anorexigenic, e.g. cholecystokinin, CCK and cocaine- and amphetamine-regulated transcript, CART) hormones. Cunners (Tautogolabrus adspersus) survive the winter in shallow coastal waters by entering a torpor-like state, during which they forgo feeding. In order to better understand the mechanisms regulating appetite/fasting in these fish, quantitative real-time PCR was used to measure transcript expression levels of four appetite-regulating hormones: NPY, CART, orexin and CCK in the forebrain (hypothalamus and telencephalon) and CCK in the gut of fed, short-term summer fasted, and natural winter torpor cunners. Summer fasting induced a decrease in hypothalamic orexin levels and telencephalon NPY, CART and CCK mRNA levels. All brain hormone mRNA levels decreased during natural torpor as compared to fed summer fish. In the gut, CCK expression levels decreased during summer fasting. These results indicate that, in cunner, orexin, NPY, CART and CCK may play a role in appetite regulation and might mediate different physiological responses to short-term summer fasting and torpor-induced long-term fasting.

Long-term effects of extreme weather events and eutrophication on the fish community of shallow Lake Peipsi (Estonia/Russia)

The fish kill in lake Peipsi (Estonia/Russia) during the extraordinarily hot summer of 2010 evoked an investigation into the effects of environmental extremes and long-term eutrophication on the fish community of the lake. Current data on lake Peipsi indicate that temperature extremes and synergistic interactions with eutrophication have led to a radical restructuring of the fish community. Commercial landings of lake smelt, Osmerus eperlanus eperlanus m. spirinchus (Pallas), the previous dominant species of the fish community, have decreased dramatically since the 1930s, these declines being coupled with summer heat waves coinciding with low water levels. Gradual decline in smelt stock and catches was significantly related to a decline of near-bottom oxygen conditions and to a decrease in water transparency. The first documented fish kill in 1959 occurred only in the southern, most shallow and eutrophic lake (lake Pihkva). Recently, summer fish kill have become more frequent, involving larger areas of the lake. In addition to the cold-water species, e.g. smelt and vendace Coregonus albula (L.), the abundance of bottom-dwelling fishes such as ruffe Gymnocephalus cernuus (L.) and juvenile fish have significantly decreased after the 2010 heat wave probably due to hypoxia and warm water temperatures. This study showed that fish community structure in large shallow lakes may be very vulnerable to water temperature increases, especially temperature extremes in combination with eutrophication.

Effect of seasonal changes on the gelling properties of farmed sea bass (Dicentrarchus labrax)

The effect of seasonal changes (summer versus winter) upon the quality of heat-induced gel products from farmed sea bass (Dicentrarchus labrax) was studied. Moreover, the effect of microbial transglutaminase addition (0.5%, w/w) on the quality was assessed. Fat content of gel products attained from sea bass grown during summer was higher (12.0% versus 7.6%). Textural properties of the gels from the summer fish were higher. Protein of gels from winter fish was less soluble in SDS + DTT (sodium dodecyl sulphate, SDS; dithiothreitol, DTT). Contrary to some literature, higher fat level may have played a protective role during processing. Accordingly, season via fat content variation had a strong effect on the gelling ability of heat-induced gels prepared from sea bass.

Maximum stream temperature and the occurrence, abundance, and behavior of steelhead trout (Oncorhynchus mykiss) in a southern California stream

To determine individual and population responses of steelhead trout (Oncorhynchus mykiss) to stream temperature, we sampled summer stream temperature and juvenile steelhead occurrence, abundance, and behavior in a small stream near the species' southern limit. Maximum annual stream temperature (22.3–33.1 °C) exerted a strong threshold effect on steelhead occurrence. Steelhead persisted through summer in all pools that did not exceed 30 °C, but the probability of persistence rapidly decreased to zero at warmer sites. Below the threshold for fish persistence, thermal effects on steelhead abundance were inconsistent, with temperature receiving strong support for inclusion in models of fish abundance at the beginning of summer but weak support for inclusion in models of end-of-summer abundance and summer fish loss. Thermal refugia that would allow steelhead to behaviorally thermoregulate were rare. In response to elevated stream temperatures, steelhead reduced foraging and agonistic activity, presumably to minimize energetic costs. In anticipation of the effects of global climate trends on stream temperature, management actions that limit or reduce stream heating will be critical for conservation of steelhead populations in habitats approaching thermal tolerance thresholds.

Zooplankton of an urban coastal lagoon: composition and association with environmental factors and summer fish kill

Zooplankton may be regarded as a sensitive tool for monitoring environmental variations in coastal lagoons due to their ability to immediately react to changes in the water column trophic features and salinity levels. As a coastal lagoon with a broad history of anthropic influence, Rodrigo de Freitas Lagoon (Rio de Janeiro, Brazil) is widely used for water sports and artisanal fishing. The present study aimed to expand the knowledge base about zooplankton in the Rodrigo de Freitas Lagoon by assessing the composition and time-spatial distribution of the major zooplankton groups. Samples were collected fortnightly from at four distinct sampling points August 2001 to July 2002. At each point, salinity, temperature, dissolved oxygen and water transparency were measured. During the study period, the lagoon behaved as an spatially homogeneous environment in what regards the abiotic variables. However, all these variables showed significant differences along the time, mainly related to seasonality (air temperature and rainy and dry periods). The zooplankton community showed low taxonomic richness, with the predominance of species commonly found in coastal lagoons, especially with mesohaline conditions, as well as those found in estuaries. An interesting fact was the rise in zooplankton abundance at all sampling points right after a fish kill event. Such increase was caused mainly by the Brachionus plicatilis O.F. Muller 1786 species. Thus, the zooplankton community was affected by physical and chemical factors, mainly by the dissolved oxygen decline event and variations in the influx of seawater into the lagoon. In addition, phytoplankton availability and fish predation pressure were suggested as important regulating factors of the zooplankton community.

DIFFERENT THERMOSTABILITY OF COLLAGENS FROM SCALE OF CARP (CYPRINUS CARPIO) IN WINTER AND SUMMER

The acid-soluble collagens (ASC) were isolated from scale of carp (Cyprinus carpio) caught in the winter and summer. Thermal denaturing behaviors of collagens were investigated by the increased pepsin digestibility and by changes in circular dichroism (CD) spectrum upon heating. The denaturation temperature of collagen were estimated from the half decrease of b component upon heating and subsequent pepsin digestion. Collagen from scales in the winter (w-scale ASC) exhibited a transition curve with a midpoint of 31.6C, which was 1.8C below that of scale collagen from the summer fish (s-scale ASC). The CD measurement results showed the melting temperature of s-scale ASC was higher than that of w-scale ASC by 3C. It was concluded s-scale ASC was more stable than the equivalent from the winter season. The research offered the fundamental information on the carp scale collagens that may be utilized in the food, cosmetic and pharmaceutical industries.

Study on the properties of gelatins from skin of carp ( Cyprinus carpio) caught in winter and summer season

The objective of this work was to compare the physiochemical (molecular weight distribution and amino acid composition) and rheological (viscosity property, gel strength and melting point) properties of gelatins from skins of carp caught in winter to those obtained for the summer equivalents. Gelatins from winter and summer fish skins were extracted at 60, 70 and 80 °C. SDS-PAGE patterns for gelatins extracted under the same conditions showed that the degradation of gelatins from winter fish skins were more severe than that of the summer ones. The imino acid contents of the winter and summer gelatins extracted at 60 °C were very similar, showing 190 and 188 residues/1000 residues, respectively. The gelatins from summer fish presented higher melting points and gel strengths, as well as better viscosity properties than the winter equivalents (P < 0.05). The differences in the rheological properties between winter and summer gelatins may be explained by different thermostability of interstitial collagen molecules (from which gelatins were derived) in the two seasons.

Fish Assemblages in Nearshore Habitats of Prince William Sound, Alaska

We sampled fish at eight locations in western Prince William Sound (PWS), Alaska, in April, July, and September 2006, and July 2007, to identify species assemblages and habitat use. At each location, fish were sampled with a 37-m long variable mesh beach seine in three nearshore habitats: bedrock outcrops, eelgrass meadows, and cobble beaches with kelp. A total of 49,060 fish representing 45 species were captured in 95 beach seine hauls. Catch-per-unit-effort (CPUE, all species) did not differ by season but did differ by habitat type–CPUE was greater in eelgrass and kelp than in bedrock. Seasonal pulses in catch were evident for some species; pink salmon were captured only in spring and summer, Pacific herring only in summer and fall, and capelin only in fall. Species richness was greater in summer (34) than in spring (23) or fall (28), and greater in eelgrass (34) than in bedrock (22) or kelp (33). Species that were good discriminators among seasonal collections were pink salmon, saffron cod, crescent gunnel, and Pacific herring, whereas species that were good discriminators among habitat collections were crescent gunnel, tubesnout, bay pipefish, saffron cod, and Arctic shanny. Of the most abundant species captured, most were juveniles based on estimated size at maturity. The summer fish assemblage in western PWS has changed over the last 20 years, especially with the appearance in large numbers of saffron cod. Sites in this study can be monitored periodically to track future changes in fish assemblages and habitat that may result from local and regional human disturbance.

Expression of GnRH genes is elevated in discrete brain loci of chum salmon before initiation of homing behavior and during spawning migration

Our previous studies suggested the importance of gonadotropin-releasing hormones (GnRHs) for initiation of spawning migration of chum salmon, although supporting evidence had been not available from oceanic fish. In farmed masu salmon, the amounts of salmon GnRH (sGnRH) mRNAs in the forebrain increased in the pre-pubertal stage from winter through spring, followed by a decrease toward summer. We thus hypothesized that gene expression for GnRHs in oceanic chum salmon changes similarly, and examined this hypothesis using brain samples from winter chum salmon in the Gulf of Alaska and summer fish in the Bering Sea. They were classified into sexually immature and maturing adults, which had maturing gonads and left the Bering Sea for the natal river by the end of summer. The absolute amounts of GnRH mRNAs were determined by real-time PCRs. The amounts of sGnRH mRNA in the maturing winter adults were significantly larger than those in the maturing summer adults. The amounts of sGnRH and chicken GnRH mRNAs then peaked during upstream migration from the coast to the natal hatchery. Such changes were observed in various brain loci including the olfactory bulb, terminal nerve, ventral telencephalon, nucleus preopticus parvocellularis anterioris, nucleus preopticus magnocellularis and midbrain tegmentum. These results suggest that sGnRH neurons change their activity for gonadal maturation prior to initiation of homing behavior from the Bering Sea. The present study provides the first evidence to support a possible involvement of neuropeptides in the onset of spawning migration.

Spatial ecology of adult muskellunge (Esox masquinongy) in the urban Ottawa reach of the historic Rideau Canal, Canada

The Rideau Canal in Canada was constructed in the 1800s to enable strategic military and commercial transport in eastern Ontario between Lake Ontario and the Ottawa River. Today, the Rideau Canal is managed by Parks Canada and remains an engineered ecosystem, particularly in the urban Ottawa reach (7 km long) where it is largely channelized and is partially drained during the winter to protect canal infrastructure (i.e., locks) and provide recreational opportunities (i.e., skating). Interestingly, the Ottawa reach of the Rideau Canal supports one of the few wild urban muskellunge (Esox masquinongy) fisheries in North America supported by natural reproduction. However, little is known about the spatial ecology, residency patterns or survival of this apex freshwater predator in urban environments or canal systems. Radio-telemetry was used to study adult (N = 10; total size range of 64 to 122 cm) muskellunge in the Ottawa reach of the Rideau Canal over a multiple year period. The seasonal movements and home range of muskellunge were greatest during spring corresponding with the period when water levels in the canal were raised and muskellunge were presumably seeking out suitable spawning habitats following ice-off. During the summer fish ranged throughout the entire 7 km urban reach of the canal and by early fall muskellunge moved to the deeper parts of the reach and became largely restricted to an embayment known as Dow's Lake prior to the lowering of the canal where they stayed for the entire winter. The same pattern of seasonal movements persisted across several years. None of the tagged muskellunge were stranded from the fall canal drainage. One fish emigrated from the urban reach of the canal during the study period, moving upstream through a lock to an exurban reach. In addition, a single tagged fish died from a winterkill event. Environmental influences such as seasonality and water depth (associated with canal operations) are believed to be the primary mechanisms contributing to habitat selection and movement patterns of muskellunge in this reach of the Rideau Canal. Data on the spatial ecology of muskellunge in the urban reaches of the Rideau Canal will inform management of this unique population of fish to ensure that the historic and ecological values are balanced to preserve this iconic Canadian natural heritage site and its biota.

Sinoatrial tissue of crucian carp heart has only negative contractile responses to autonomic agonists

BackgroundIn the anoxia-tolerant crucian carp (Carassius carassius) cardiac activity varies according to the seasons. To clarify the role of autonomic nervous control in modulation of cardiac activity, responses of atrial contraction and heart rate (HR) to carbacholine (CCh) and isoprenaline (Iso) were determined in fish acclimatized to winter (4°C, cold-acclimated, CA) and summer (18°C, warm-acclimated, WA) temperatures.ResultsInhibitory action of CCh was much stronger on atrial contractility than HR. CCh reduced force of atrial contraction at an order of magnitude lower concentrations (EC50 2.75-3.5·10-8 M) in comparison to its depressive effect on HR (EC50 1.23-2.02·10-7 M) (P < 0.05) without differences between winter and summer acclimatized fish. Inhibition of nitric oxide synthase with 100 μM L-NMMA did not change the response of the sinoatrial tissue to CCh. Reduction of atrial force was associated with a strong shortening of action potential (AP) duration to ~50% (48 ± 10 and 50 ± 6% for CA and WA fish, respectively) and 11% (11 ± 3 and 11 ± 2% for CA and WA fish, respectively) of the control value at 3·10-8 M and 10-7 M CCh, respectively (P < 0.05). In atrial myocytes, CCh induced an inwardly rectifying K+ current, IK,CCh, with an EC50 value of 3-4.5·10-7 M and inhibited Ca2+ current (ICa) by 28 ± 8% and 51 ± 6% at 10-7 M and 10-6 M, respectively. These currents can explain the shortening of AP. Iso did not elicit any responses in crucian carp sinoatrial preparations nor did it have any effect on atrial ICa, probably due to the saturation of the β-adrenergic cascade in the basal state.ConclusionIn the crucian carp, HR and force of atrial contraction show cardio-depressive responses to the cholinergic agonist, but do not have any responses to the β-adrenergic agonist. The scope of inhibitory regulation by CCh is increased by the high basal tone of the adenylate cyclase-cAMP cascade. Higher concentrations of CCh were required to induce IK,CCh and inhibit ICa than was needed for CCh's negative inotropic effect on atrial muscle suggesting that neither IK,CCh nor ICa alone can mediate CCh's actions but they might synergistically reduce AP duration and atrial force production. Autonomic responses were similar in CA winter fish and WA summer fish indicating that cardiac sensitivity to external modulation by the autonomic nervous system is not involved in seasonal acclimatization of the crucian carp heart to cold and anoxic winter conditions.

Summer fish communities in northern Gulf of Mexico estuaries: Indices of ecological condition

We used fish community data from trawl samples collected from >100 estuaries, bayous, and coastal lagoons of the Louisianan Biogeographic Province (Gulf of Mexico) to develop indicators of large-scale ecological condition. One data set, from which we derived reference values for fish community indicators, was based on bottom trawl samples collected from 367 randomly located sites during the summers of 1992–1994. A second trawl data set with similar geographic scope from 2000 to 2004 was used to test the robustness of the indicators derived from the reference data set to new data. We constructed a fish community index (FCI) from three basic indicators: number of species per trawl, total abundance per trawl, and an index of trophic balance among three common feeding guilds. The FCI was not correlated with salinity over a range from freshwater to marine and hypersaline conditions (0–52 psu). Direct correlations between the index and environmental variables generally were weak, although some were significant ( p < 0.05). The FCI was negatively correlated with water clarity (secchi depth), water column depth, and sediment toxicity; correlations of the FCI with pH, sediment organic carbon, and sediment silt + clay content were positive. There was a hyperbolic relationship between dissolved oxygen and maximum values of the index, and no significant correlation with watershed land cover at the whole-estuary or estuary-complex scale. Values of all indicators increased between the two time periods. The FCI is a broad indicator of ecological condition for estuaries within the Louisianan Province, with data aggregated at scales ranging from large estuaries to the entire region. Sample density was insufficient to judge performance of the indicators or index at smaller scales.

Hydrogen sulfide production and volatilization in a polymictic eutrophic saline lake, Salton Sea, California

The Salton Sea is a large shallow saline lake located in southern California that is noted for high sulfate concentrations, substantial algal productivity, and very warm water column temperatures. These conditions are well-suited for sulfide production, and sulfide has been implicated in summer fish kills, although no studies have been conducted to specifically understand hydrogen sulfide production and volatilization there. Despite polymictic mixing patterns and relatively short accumulation periods, the amount of sulfide produced is comparable to meromictic lakes. Sulfide levels in the Salton Sea reached concentrations of 1.2 mmol L − 1 of total free sulfide in the hypolimnion and 5.6 mmol L − 1 in the sediment pore water. Strong winds in late July mixed H 2S into the surface water, where it depleted the entire water column of dissolved oxygen and reached a concentration of 0.1 mmol L − 1 . Sulfide concentrations exceeded the toxicity threshold of tilapia ( Oreochromis mossambicus) and combined with strong anoxia throughout the water column, resulted in a massive fish kill. The mixing of sulfide into the surface waters also increased atmospheric H 2S concentrations, reaching 1.0 μmol m − 3 . The flux of sulfide from the sediment into the water column was estimated to range from 2–3 mmol m − 2 day − 1 during the winter and up to 8 mmol m − 2 day − 1 during the summer. Application of the two-layer model for volatilization indicates that up to 19 mmol m − 2 day − 1 volatilized from the surface during the mixing event. We estimate that as much as 3400 Mg year − 1 or approximately 26% of sulfide that diffused into the water column from the deepest sediments may have been volatilized to the atmosphere.

Overwintering ability of young-of-the-year bluefish Pomatomus saltatrix: effect of ration and cohort of origin on survival

Bluefish Pomatomus saltatrix have experienced declines in recruitment and adult abun- dance along the US East Coast since the mid-1980s. At the onset of winter, young-of-the-year (YOY) bluefish exhibit a multimodal size distribution including larger, spring-spawned fish (spring cohort) and smaller, summer-spawned fish (summer cohort). Declines in the adult stock appear to coincide with declines in recruitment success of the summer cohort. We investigated the hypothesis that poor recruitment success of the summer cohort results from size-selective winter mortality. Winter meso- cosm experiments were conducted to examine the effects of cohort of origin (spring vs. summer) and food availability (fed vs. unfed) on winter survival of YOY bluefish. Spring fish entered winter with sig- nificantly greater lipid reserves than summer fish. When fed, both cohorts stored lipids during late fall, depleted lipid reserves during winter, and experienced high overwinter survival. When starved, both cohorts mobilized lipids from multiple depots (liver, viscera, white muscle, red muscle, skin) and sum- mer fish experienced starvation mortality ~6 wk prior to spring fish. Although summer fish were more susceptible to winter starvation than spring fish, their starvation endurance (>90% survival probabil- ity after 120 d) appeared more than adequate to survive natural winter conditions. Interestingly, spring fish suffered a brief mortality event during January when water temperatures dropped briefly below 6°C, suggesting that larger individuals are less tolerant of acute cold stress. The remarkable starvation endurance of summer-spawned bluefish, coupled with their capacity for rapid lipid storage during fall and reduced rates of lipid depletion at low temperatures, implies that members of this cohort are physiologically well-equipped to survive their first winter of life. Our findings are inconsistent with the hypothesis that winter starvation accounts for decreased recruitment of the summer cohort to the western Atlantic stock.

Summer Fish Community of the Coastal Northern Gulf of Mexico: Characterization of a Large‐Scale Trawl Survey

AbstractFish were collected by otter trawl at 367 sites from 119 coastal water bodies in the northern Gulf of Mexico, ranging from north‐central Florida to the Rio Grande, Texas, during the summer months of 1992‐1994. The fish were identified and enumerated, and tissue contaminants (in fillets), and external abnormalities (body, buccal, branchial, and ocular) were determined. Results were compared for sites east and west of the Mississippi River, within the Mississippi River, seven estuarine categories, and over 3 years. Approximately 31,000 individual fish were captured, representing 100 genera and 141 species. Thirteen species comprised 91% of the total abundance; abundances of 128 species were 1% or less. Pinfish Lagodon rhomboides, Atlantic croaker Micropogonias undulatus, Gulf menhaden Brevoortia patronus, and bay anchovy Anchoa mitchilli were the more dominant species. Fourteen species were collected from at least 10% of the sites; 85 species were captured at no more than 1% of the sites. Atlantic croakers and hardhead catfish Ariopsis felis were collected more frequently (&gt;50% of sites). Indices of community structure from the full data set ranged as follows: 2.8 for the Shannon‐Wiener index, 0.90 for Simpson's index, and 0.56 for Pielou's evenness index. Chemical contaminants (predominantly total mercury and polychlorinated biphenyls [PCBs]) exceeded federal risk‐based consumption guidelines in 2.7% of fillets from four species. External abnormalities occurred on 17 species and approximately 1% of the total number of fish examined. Community structural properties varied temporally across years and spatially over estuarine categories and between areas east and west of the Mississippi River. Four distinct assemblages were identified by cluster analysis of species abundance. Cluster membership was associated with salinity, depth, dissolved oxygen, water clarity, and geographic area.

Oxidative stress and antioxidant defense responses by goldfish tissues to acute change of temperature from 3 to 23 °C

The effects of a rapid transfer from a low (3 °C) to a warm (23 °C) temperature on oxidative stress markers and antioxidant defenses were studied in the brain, liver and kidney of the goldfish, Carassius auratus. Cold-acclimated fish were acutely moved to 23 °C and sampled after 1, 6, 12, 24, 48 or 120 h of warm temperature exposure. Lipid peroxide levels increased quickly during the first few hours at 23 °C, but thiobarbituric acid-reactive substances changed little. Protein carbonyl content was reduced by 20–40% in the liver over the entire experimental course, but increased transiently in the kidney. The content of high-molecular mass thiols decreased by two-thirds in the brain and was affected slightly in other organs. By contrast, total low-molecular mass thiols (e.g. glutathione and others) increased transiently. Activities of the primary antioxidant enzymes—superoxide dismutase and catalase—were generally unaffected in goldfish organs, whereas glutathione-dependent enzymes were elevated in the brain and kidney after 24–48 h at 23 °C. Glutathione peroxidase increased by 1.5–2.3-fold and glutathione- S-transferase by 1.7-fold. Hence, a short-term exposure to warm temperature disturbed several oxidative stress markers, but only slightly affected the activities of antioxidant enzymes. However, comparison of the current data for cold-acclimated winter fish with the same parameters in summer fish suggests that longer exposure to high ambient temperature requires the enhancement of activities of glutathione-dependent enzymes for maintaining the steady-state levels lipid peroxidation and protein oxidation in goldfish tissues.

Behavioural observations of foraging minke whales (Balaenoptera acutorostrata) in the outer Moray Firth, north-east Scotland

The minke whale (Balaenoptera acutorostrata) is a widespread, opportunistic species showing spatial and seasonal variations in diet according to local availability of prey. Although previous research has been conducted on the foraging strategies of this small rorqual whale, in terms of prey aggregation and assimilation, relatively little has been published on the foraging association of this species with coastal seabirds. Over the past five years, minke whales occurring along the outer coastline of the southern Moray Firth in north-east Scotland during the summer and autumn months have been recorded foraging in the presence of seabirds, such as kittiwakes (Rissa tridactyla), herring gulls (Larus argentatus), guillemots (Uria aalge) and shearwaters, which form dense feeding rafts at the water's surface. The formation of bird rafts notably occurs independently of the presence of B. acutorostrata, believed instead to be the successive result of prey concentrated at the surface by predatory schooling fish from below rather than by activities of the whales themselves. In this area of the North Sea, schooling mackerel (Scomber scombrus) constitute the most significant component of the summer fish biomass and are believed to perform the role of compacting targeted sandeel (Ammodytes spp.) prey into concentrated bait balls almost exclusively. The resulting ball of prey is consequently available to the foraging whale, which can be seen opportunistically utilizing successive bird rafts rather than expending unnecessary energy corralling the Ammodytes prey by traditional, active entrapment methods. The role of the mackerel in increasing both the rate and density of sandeel ball formation (as indicated by the presence and activity of associated bird rafts) is therefore thought to be very significant in this inshore Scottish location. Changes in oceanographic variables such as water temperature have been directly correlated with the migration of S. scombrus. The observed inter-annual variability in B. acutorostrata distribution in the outer Moray Firth may subsequently be related to the respective distribution and abundance of these migratory, pelagic fish species.

Associations between fish assemblages and environmental factors for Mediterranean‐type rivers during summer

The structure of summer fish assemblages was examined along longitudinal gradients in 31 Mediterranean‐type rivers of the middle Guadiana basin (south‐west Iberian Peninsula), using data from 157 sites including small streams to deep rivers. An ordination analysis, based on 16 variables, was applied to species presence, using principal component and canonical correspondence analysis. The results for the habitat data were compared with those for the biological data using a Mantel analysis, and the agreement was highly significant. Spatial structure was considered by partitioning the total variability among the environmental and geographical variables. The fish assemblages showed longitudinal zoning during the summer, with species distributed over gradients of habitat size (depth), water quality (current and physico‐chemical variables), and cover (substratum and vegetation), according to their adult size and life history. The size of the habitat that remained available in summer had the greatest biological effect, being the most important factor explaining fish species distribution and assemblage structure during this stressing season. Strictly spatial variation was low, but there was still a high residual variation. Habitat associations and life‐history strategies are discussed for native and exotic species.

Seasonal and diel changes of dissolved oxygen in a hypertrophic prairie lake

AbstractHumboldt Lake, a hypertrophic prairie lake typical of many found on the Great Plains of North America, is usually ice‐covered from early November to about mid‐May. The lake is an important recreational fishery, now mainly stocked with walleye. It has a high potential risk of experiencing fish kills because of the very large cyanobacterial blooms that develop in it, the high rates of algal and bacterial production and the high concentrations of ammonia (NH3‐N) and dissolved organic matter. Following the collapse of cyanobacterial blooms, shallow prairie lakes are known to undergo periods of anoxia that can lead to summer fish kills. In some of the lakes, anoxia forms during the long period of ice cover, causing winter fish kills. Two years of seasonal and diel data (total phosphorus, dissolved oxygen (DO), NH3‐N and chlorophyll‐a concentrations, and bacterial production) were analysed in this study to assess why significant fish kills did not occur during this period or during the ≈ 30 years of records from Saskatchewan Environment. Humboldt Lake did not become anaerobic, either following the collapse of the cyanobacterial bloom or under ice cover, indicating that the oxygen (O2) influx (strong mixing) and production processes were greater than the microbial and chemical O2 demands, both over seasonal and diel time scales. Several published risk threshold criteria to predict the probability of summer and/or winter fish kills were applied in this study. The threshold criteria of maximum summer chlorophyll and maximum winter NH3‐N concentrations indicated that a summer fish kill was unlikely to occur in this hypertrophic prairie lake, provided its water quality remained similar to that during this study. Similarly, the threshold criteria of initial DO storage before ice cover and the rate of O2 depletion under ice cover also indicated a winter fish kill was unlikely. However, recent development in the watershed might have resulted in significant water quality deterioration and the winter fish kill that occurred in 2005.

NO ARREST NEEDED TO PREVENT CARDIAC ARREST

Most vertebrates die within a few minutes when deprived of molecular oxygen(anoxia) because the heart and brain depend on a continuous supply of oxygen. However, some cold-blooded vertebrates are remarkably anoxia-tolerant and can survive for months without oxygen. One of the keys to surviving prolonged anoxia is being able to balance energy supply and demand. One way to achieve this is to reduce energy-consuming processes of cells to a level that can be supported by the decreased amount of energy available from anaerobic respiration. Since ion pumping is energetically costly, energy can potentially be conserved by reducing the density of cellular membrane ion transport channels in excitable tissues such as the heart. Matti Vornanen and Vesa Paajanen of the University of Joensuu, Finland, set out to test this `channel arrest' hypothesis in the crucian carp (Carassius carassius), an extremely anoxia-tolerant fish that spends a third of its life in anoxic water under the ice of shallow Finno-Scandinavian ponds.The oxygen content of the water in the ponds where crucian carp live is too low for the fish to produce energy aerobically for five and a half months throughout the winter, so carp need to conserve energy during winter. Vornanen and Paajenen reasoned that since L-type Ca2+ channels regulate the force of heart muscle contraction through changes in the amount of free intracellular Ca2+, these channels are good targets for channel arrest. They hypothesized that, if crucian carp use channel arrest to conserve energy during anoxia in winter, the number of dihydropyridine receptors(subunit of the L-type Ca2+ cardiac channel that triggers channel opening) and density of L-type Ca2+ current should both be decreased in winter-captured, anoxic carp compared with summer-captured,normoxic animals.Vornanen and Paajanen captured wild carp monthly throughout an entire year and measured the number of dihydropyridine receptors and the density of L-type Ca2+ current in the ventricles of the carp's hearts to see how these changed with the seasons. Unexpectedly, they found that the number of dihydropyridine receptors in winter fish was the same as in summer fish. Thus,cardiac L-type Ca2+ channels were not downregulated by seasonal anoxia in the natural environment, suggesting that carp do not use differential expression of the Ca2+ channel protein to save energy in ion pumping or in reduced cardiac contractility during anoxia. This finding, in conjunction with the team's earlier finding that inward rectifier K+ current density of crucian carps' heart cells is also unaffected by prolonged anoxia, led Vornanen and Paajanen to conclude that crucian carp do not use channel arrest as an energy-conserving mechanism in the heart during prolonged anoxia.However, Vornanen and Paajanen did observe a 6.1-fold reduction in Ca2+ current in winter compared with summer fish, indicating that low temperatures depress Ca2+ current. They suggest that when temperatures drop, the resulting decreased Ca2+ current may allow for sufficient savings in ATP-dependent ion pumping and reduced cardiac contractility to attain a balance between energy supply and demand, providing protection against anoxia.